Controlled Atmosphere Storage Adel A. Kader Kader is with the Department of Pomology, University of California, Davis, CA. He is now retired. Introduction Controlled atmosphere (CA) storage involves maintaining an atmospheric composition that is different from air composition (about 78% N 2, 21% O 2, and 0.03% CO 2 ); generally, O 2 below 8% and CO 2 above 1% are used. Atmosphere modification should be considered as a supplement to maintenance of optimum ranges of temperature and RH for each commodity in preserving quality and safety of fresh fruits, ornamentals, vegetables, and their products throughout postharvest handling. This chapter gives an overview of responses to CA; specific CA considerations are given in individual commodity summaries. Biological Basis of CA Effects Exposure of fresh horticultural crops to low O 2 and/or elevated CO 2 atmospheres within the range tolerated by each commodity reduces their respiration and ethylene production rates; however, outside this range respiration and ethylene production rates can be stimulated, indicating a stress response. This stress can contribute to incidence of physiological disorders and increased susceptibility to decay. Elevated CO 2 -induced stresses are additive to and sometimes synergistic with stresses caused by low O 2, physical or chemical injuries, and exposure to temperatures, RH, and/or C 2 H 4 concentrations outside the optimum range for the commodity. The shift from aerobic to anaerobic respiration depends on fruit maturity and ripeness stage (gas diffusion characteristics), temperature, and duration of exposure to stress-inducing concentrations of O 2 and/or CO 2. Up to a point, fruits and vegetables are able to recover from the detrimental effects of low O 2 and high CO 2 stresses (fermentative metabolism) and resume normal respiratory metabolism upon transfer to air. Plant tissues have the capacity for recovery from the stresses caused by brief exposure to fungistatic atmospheres (>10% CO 2 ) or insecticidal atmospheres (<1% O 2 and/or 40 to 80% CO 2 ). Postclimacteric fruits are less tolerant and have lower capacity for recovery following exposure to reduced O 2 or elevated CO 2 levels than preclimacteric fruits. The speed and extent of recovery depend on duration and levels of stresses and underlying, metabolically driven cellular repair. Elevated-CO 2 atmospheres inhibit activity of ACC synthase (key regulatory site of ethylene biosynthesis), while ACC oxidase activity is stimulated at low CO 2 and inhibited at high CO 2 concentrations and/or low O 2 levels. Ethylene action is inhibited by elevated CO 2 atmospheres. Optimum atmospheric compositions retard chlorophyll loss (green color), biosynthesis of carotenoids (yellow and orange colors) and anthocyanins (red and blue colors), and biosynthesis and oxidation of phenolic compounds (brown color). Controlled atmospheres slow down the activity of cell wall degrading enzymes involved in softening and enzymes involved in lignification, leading to toughening of vegetables. Low O 2 and/or high CO 2 atmospheres influence flavor by reducing loss of acidity, starch to sugar conversion, sugar interconversions, and biosynthesis of flavor volatiles. When produce is kept in an optimum atmosphere, retention of ascorbic acid and other vitamins are retained, resulting in better nutritional quality. Severe stress CA conditions decrease cytoplasmic ph and ATP levels and reduce pyruvate dehydrogenase activity, while pyruvate decarboxylase, alcohol dehydrogenase, and lactate dehydrogenase are induced or activated. This causes accumulation of acetaldehyde, ethanol, ethyl acetate, and/or lactate, which may be detrimental to the commodities if they are exposed to stress CA conditions beyond their tolerance. Specific responses to CA depend on cultivar, maturity and ripeness stage, storage temperature and duration, and in some cases ethylene concentrations.
N 2 is an inert component of CA. Replacing N 2 with argon or helium may increase diffusivity of O 2, CO 2, and C 2 H 4, but they have no direct effect on plant tissues and are more expensive than N 2 as a CA component. Super-atmospheric levels of O 2 up to about 80% may accelerate ethylene-induced degreening of nonclimacteric commodities and ripening of climacteric fruits, respiration and ethylene production rates, and incidence of some physiological disorders (such as scald on apples and russet spotting on lettuce). At levels above 80% O 2, some commodities and postharvest pathogens suffer from O 2 toxicity. Use of super-atmospheric O 2 levels in CA will likely be limited to situations in which they reduce the negative effects of fungistatic, elevated CO 2 atmospheres on commodities that are sensitive to CO 2 -induced injury. Beneficial Effects of CA (Optimum Composition for the Commodity) A Summary Retardation of senescence (including ripening) and associated biochemical and physiological changes, particularly slowing down rates of respiration, ethylene production, softening, and compositional changes. Reduction of sensitivity to ethylene action at O 2 levels <8% and/or CO 2 levels >1%. Alleviation of certain physiological disorders such as chilling injury of avocado and some storage disorders, including scald of apples. CA can have a direct or indirect effect on postharvest pathogens (bacteria and fungi) and consequently decay incidence and severity. For example, CO 2 at 10 to 15% significantly inhibits development of botrytis rot on strawberries, cherries, and other perishables. Low O 2 (<1%) and/or elevated CO 2 (40 to 60%) can be a useful tool for insect control in some fresh and dried fruits, flowers, and vegetables and in dried nuts and grains. Detrimental Effects of CA (Above or Below Optimum Composition for the Commodity) A Summary Initiation and/or aggravation of certain physiological disorders such as internal browning in apples and pears, brown stain of lettuce, and chilling injury of some commodities. Irregular ripening of fruits, such as banana, mango, pear, and tomato, can result from exposure to O 2 levels below 2% and/or CO 2 levels above 5% for >1 mo. Development of off flavors and off odors at very low O 2 concentrations (as a result of anaerobic respiration) and very high CO 2 levels (as a result of fermentative metabolism). Increased susceptibility to decay when the fruit is physiologically injured by too low O 2 or too high CO 2 concentrations. Commercial Application of CA Storage Several refinements in CA storage have been made in recent years to improve quality maintenance. These include creating nitrogen by separation from compressed air using molecular sieve beds or membrane systems, low-o 2 (1.0 to 1.5%) storage, low-ethylene (<1 µl L -1 ) CA storage, rapid-ca (rapid establishment of optimal levels of O 2 and CO 2 ), and programmed- (or sequential-) CA storage for example, storage in 1% O 2 for 2 to 6 weeks followed by storage in 2 to 3% O 2 for the remainder of the storage period. Other developments, which may expand use of atmospheric modification during transport and distribution, include improved technologies for establishing, monitoring, and maintaining CA using edible coatings or polymeric films with appropriate gas permeability to create a desired atmospheric composition around and within the commodity. Modified atmosphere packaging (MAP) is widely used in marketing fresh-cut produce. Applications of CA to cut flowers are very limited because decay caused by Botrytis cinerea is often a limiting factor to postharvest life, and fungistatic CO 2 levels damage flower petals and/or associated stem and leaves. Also, it is less expensive to treat flowers with anti-ethylene chemicals than to use CA to minimize ethylene action.
Commercial use of CA storage is greatest on apples and pears worldwide, less on cabbages, sweet onions, kiwifruits, avocados, persimmons, pomegranates, and nuts and dried fruits and vegetables (table 1). Atmospheric modification during long-distance transport is used with apples, asparagus, avocados, bananas, broccoli, cane berries, cherries, figs, kiwifruits, mangos, melons, nectarines, peaches, pears, plums, and strawberries. Continued technological developments in the future to provide CA during transport and storage at a reasonable cost (positive benefit/cost ratio) are essential to greater applications on fresh horticultural commodities and their products.
Table 1. Classification of horticultural crops according to their CA storage potential at optimum temperatures and RH. -------------------------------------------------------------------------------------------------------------------------------- Storage duration Commodities -------------------------------------------------------------------------------------------------------------------------------- months >12 Almond, Brazil nut, cashew, filbert, macadamia, pecan, pistachio, walnut, dried fruits and vegetables 6 to 12 Some cultivars of apples and European pears 3 to 6 Cabbage, Chinese cabbage, kiwifruit, persimmon, pomegranate, some cultivars of Asian pears 1 to 3 Avocado, banana, cherry, grape (no SO 2 ), mango, olive, onion (sweet cultivars), some cultivars of nectarine, peach and plum, tomato (mature-green) <1 Asparagus, broccoli, cane berries, fig, lettuce, muskmelons, papaya, pineapple, strawberry, sweet corn, fresh-cut fruits and vegetables, some cut flowers --------------------------------------------------------------------------------------------------------------------------------
Additional Reading and Reference Material Calderon, M., and R. Barkai-Golan, eds. 1990. Food Preservation by Modified Atmospheres. CRC Press, Boca Raton, FL. El-Goorani, M.A., and N.F. Sommer. 1981. Effects of modified atmospheres on postharvest pathogens of fruits and vegetables. Hort. Rev. 3:412-461. Gorny, J., ed. 1997. Fresh-Cut Fruits and Vegetables and MAP, CA 97 Proceedings, vol. 5. Postharvest Hort. Ser. no. 19, University of California, Davis, CA. Kader, A.A. 1986. Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits and vegetables. Food Technol. 405:99-100, 102-104. Kader, A.A., ed. 1997. Fruits Other Than Apples and Pears, CA 97 Proceedings, vol. 3. Postharvest Hort. Ser. no. 17, University of California, Davis CA. Kader, A.A., D. Zagory, and E.L. Kerbel. 1989. Modified atmosphere packaging of fruits and vegetables. CRC Crit. Rev. Food Sci. Nutr. 28:1-30. Mitcham, E.J., ed. 1997. Apples and Pears, CA 97 Proceedings, vol. 2. Postharvest Hort. Ser. no. 16, University of California, Davis, CA. Raghavan, G.S.V., P. Alvo, Y. Gairepy, and C. Vigneault. 1996. Refrigerated and controlled modified atmosphere storage. In L.P. Somogyi et al., eds, Processing Fruits: Science and Technology, vol. 1, Biology, Principles and Applications, pp. 135-167. Technomic Pub. Co., Lancaster, PA. Saltveit, M.E., ed. 1997. Vegetables and Ornamentals, CA 97 Proceedings, vol. 4. Postharvest Hort. Ser. no. 18, University of California, Davis, CA. Thompson, A.K. 1998. Controlled Atmosphere Storage of Fruits and Vegetables. CAB International, Wallingford, U.K. Thompson, J.F., and E.J. Mitcham, eds. 1997. Technology and Disinfestation Studies, CA 97 Proceedings, vol. 1. Postharvest Hort. Ser. no. 15, University of California, Davis, CA. Vigneault, C., V.G.S. Raghavan, and R. Prange. 1994. Techniques for controlled atmosphere storage of fruit and vegetables. Tech. Bull 1993-18E, Agriculture Canada, Kentville, N.S. Wang, C.Y. 1990. Physiological and biochemical effects of controlled atmosphere on fruits and vegetables. In M. Calderon and R. Barkai-Golan, eds, Food Preservation by Modified Atmospheres, pp. 197-223. CRC Press, Boca Raton, FL. Weichmann, J. 1986. The effect of controlled atmosphere storage on the sensory and nutritional quality of fruits and vegetables. Hort. Rev. 8:101-127. Yahia, E.M. 1998. Modified and controlled atmosphere for tropical fruits. Hort. Rev. 22:123-183. Zagory, D., and A.A. Kader. 1989. Quality maintenance in fresh fruits and vegetables by controlled atmospheres. In J.J. Jen, ed., Quality Factors of Fruits and Vegetables: Chemistry and Technology, pp.
174-188. American Chemical Society, Washington, DC